During the mozilla-aurora cycle, the bug 1206771 didn't bit us. When we moved m-a into m-b, we had the build failing because of this bug. Maybe -Werror=unused-but-set-variable is set only on m-b and not on m-a? Anyway, this should be enabled on nightly, aurora, etc
The only code using the variable was |MOZ_DIAGNOSTIC_ASSERT(!reentrantFlag);|, and that's not supposed to have any effect on beta/release (see https://dxr.mozilla.org/mozilla-central/rev/9ed17db42e3e46f1c712e4dffd62d54e915e0fac/mfbt/Assertions.h#298) So this seems expected. But does seem like a bit of a footgun to have it cause errors like bug 1206771 upon uplift...
Component: Release Automation → MFBT
Product: Release Engineering → Core
QA Contact: bhearsum
Summary: different behavior with CXXFLAGS on m-a and m-b → Build failed after uplift due to -Werror=unused-but-set-variable + MOZ_DIAGNOSTIC_ASSERT
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